Jadeitic Pyroxene Research Articles

Neoblastic jadeitic pyroxene in Franciscan metagreywackes from Pacheco Pass, central Diablo Range, California, and implications for the inferred metamorphic P‐T trajectory

Abstract Textural, chemical, and areal relations of jadeitic pyroxene‐bearing, quartzose metagreywackes near Pacheco Pass, east‐central California Coast Ranges, indicate that: Na‐clinopyroxene is a newly generated mineral rather than clastic material; pumpellyite is a minor or nonexistent neoblastic phase in associated albite‐bearing metagreywackes, hence growth of jadeitic pyroxene must have resulted from a reaction of the sort chlorite + Na‐plagioclase + iron oxide = jadeitic pyroxene + glaucophane + lawsonite + quartz ± H2O; and, because of sluggish reaction rates, sodic plagioclase persisted in a metastable condition into the higher pressure, nearly end‐member jadeitic pyroxene + quartz P‐T field. Irregular zoning within single jadeitic pyroxene prisms and variable clino‐pyroxene/albite modes unrelated to Na‐clinopyroxene compositions are consequences of kinetics rather than reflecting equilibrium recrystallisation within a P‐T transition zone; high‐pressure overstepping of the stability boundary can ...

Petrology of the nyböite-bearing eclogite in the Donghai area, Jiangsu Province, eastern China

AbstractNyböite occurs as porphyroblasts in the Jianchang eclogite in the Donghai area, northeastern Jiangsu Province, eastern China. The Jianchang eclogite contains some inclusions of quartz after coesite in clinopyroxene, garnet and epidote. It has colourless to pale-violet pleochroism. A thin rim with violet pleochroism often develops around nyb6ite and is taramitic. It is further retrogressed by the symplectite which is mainly composed of hornblende, aegirine-augite and albite. Nyböite is associated with jadeitic pyroxene in the Jianchang eclogite, although other porphyroblastic amphiboles in other Donghai eclogites are barroisitic to katophoritic and are associated with omphacite.Fe-Mg partitioning between garnet and clinopyroxene and the presence of coesite pseudomorphs indicate P-T conditions in the Jianchang eclogite of about 740 ± 60°C and more than 28 kbar. Similar P-T conditions were estimated for other porphyroblastic amphibole-bearing eclogites in the Donghai area. Nyböite can occur in the Na-Al-Fe-rich local bulk composition under the medium to high temperature and very high-pressure conditions. Retrograde rim amphibole is poorer in NaB, variable in Si content, and richer in NaA variable than the porphyroblastic amphibole in the Donghai area. This roughly implies a P-T path where P decreases without a large decrease of T.

Neoblastic jadeitic pyroxene in Franciscan metagreywackes from Pacheco Pass, central Diablo Range, California, and implications for the inferred metamorphic P‐T trajectory

Neoblastic jadeitic pyroxene in Franciscan metagreywackes from Pacheco Pass, central Diablo Range, California, and implications for the inferred metamorphic P‐T trajectory

Precambrian blueschist terranes of the world

Blueschists are most common in post-Paleozoic orogenic belts; higher geothermal gradients and a thinner lithosphere in the Precambrian are widely thought to have disfavored blueschist formation. However, several Precambrian blueschist belts have been reported from Eurasia. On the basis of established geologic and geochronologic constraints, these belts are grouped as suspect, possible and definite Precambrian blueschists. Suspect ones include those in Alaska, in Anglesey (U.K.), in the Scandinavian Caledonides, and in Yunnan (China). Possible Precambrian blueschist belts are those in Delhi (India), in central China, in Heilongjiang (China) and in the Urals and Altai-Sayan-Tien Shan (U.S.S.R.). Definite Precambrian blueschist terranes are those in western Africa and in Aksu (China). These blueschist belts are described with emphasis on the age constraints. The recently investigated Aksu blueschist terrane is presented as an example. This high- P/ T coherent blueschist-greenschist terrane at Aksu is unconformably overlain by a thick, unmetamorphosed sedimentary sequence ranging in age from Late Sinian (ca. 600–800 Ma) through Eocambrian to Paleozoic. The basal Sinian conglomerates contain subrounded to rounded clasts of the underlying schist, clearly demonstrating the pre-Sinian age of blueschist facies metamorphism. Characteristic features of the above-mentioned blueschist belts include: 1. (1) the blueschist terranes are coherent and primary compositional-lithological layers are well preserved; mèlange belts are not common; 2. (2) protoliths include pelite, sandstone, bimodal volcanics, limestone and small quantities of pillow basalt and pelagic sediment; ultramafics are rare; 3. (3) the protoliths were subjected to multiple deformation and transitional blueschist-greenschist facies metamorphism. Typical Precambrian blueschist contains the assemblage crossitic amphibole + epidote + chlorite + albite + quartz ± actinolite); rare lawsonite, jadeitic pyroxene and aragonite have been described in few belts; 4. (4) the metamorphic facies series belongs to the high-pressure intermediate type of Miyashiro (1961). The Precambrian blueschist and other pre-Mesozoic intracontinental blueschist of Eurasia formed before, rather than during, final closure of intervening ocean basins between pairs of continents. They do not usually mark the location of terminal sutures, but are the result of earlier accretion and continental growth by subduction, underplating and imbrication of accretionary wedges including both oceanic and continent-derived elastics. The rare preservation of Precambrian blueschist is due to greenschist overprinting when the heat flow and geothermal gradient was high, rather than reflecting lack of production. Their existence in China and Africa demonstrates that requisite conditions for blueschist facies metamorphism existed at least 700 Ma ago.

Glaucophane‐bearing assemblage overprinted by greenschist‐facies metamorphism in the Variscan Kaczawa complex, Sudetes, Poland

An Early Palaeozoic (Ordovician ?) metamudstone sequence near Wojcieszow, Kaczawa Mts, Western Sudetes, Poland, contains numerous metabasite sills, up to 50 m thick. These subvolcanic rocks are of within‐plate alkali basalt type. Primary igneous phases in the metabasites, clinopyroxene (salite) and kaersutite, are veined and partly replaced by complex metamorphic mineral assemblages. Particularly, the kaersutite is corroded and rimmed by zoned sodic, sodic–calcic and calcic amphiboles. The matrix is composed of actinolite, pycnochlorite, albite (An ≤ 0.5%), epidote (Ps 27–33), titanite, calcite, opaques and, occasionally, biotite, phengite and stilpnomelane.The sodic amphiboles are glaucophane to crossite in composition with NaB from 1.9 to 1.6. They are rimmed successively by sodic–calcic and calcic amphiboles with compositions ranging from magnesioferri‐winchite to actinolite. No compositions between NaB= 0.92 and NaB= 1.56 have been ascertained.The textures may be interpreted as representing a greenschist facies overprint on an earlier blueschist (or blueschist–greenschist transitional) assemblage. The presence of glaucophane and no traces of a jadeitic pyroxene + quartz association indicate pressures between 6 and 12 kbar during the high‐pressure episode. Temperature is difficult to assess in this metamorphic event. The replacement of glaucophane by actinolite + chlorite + albite, with associated epidote, allows restriction of the upper pressure limit of the greenschist recrystallization to <8 kbar, between 350 and 450°C. The mineral assemblage representing the greenschist episode suggests the P–T conditions of the high‐pressure part of the chlorite or lower biotite zone. The latest metamorphic recrystallization, under the greenschist facies, may have taken place in the Viséan.

Controls on the first appearance of jadeitic pyroxene, northern Diablo Range, California

A reaction producing jadeitic pyroxene in metagreywackes of the northern Diablo Range has been identified on the basis of mineral distribution, isograd patterns and composition of coexisting minerals. The appearance of jadeitic pyroxene (∼Jd80) is closely followed by the disappearance of pumpellyite, which indicates that pumpellyite plays a major role in the pyroxene‐producing reaction. A new projection from hematite, lawsonite, chlorite, quartz and H2O on to the NaAlO2‐FeO‐MgO ternary confirms the role of pumpellyite in pyroxene production and suggests a reaction of the form: 1.00 pumpellyite + 0.31 chlorite + 8.71 albite + 0.70 hematite + 2.00 H2O = 8.54 jadeite + 0.57 glaucophane + 3.09 lawsonite + 5.26 quartz. Metagreywackes of the northern Diablo Range were metamorphosed under conditions of PH2O=Ptotal at 200‐300 °C and 7.5‐10.0 kbar. Despite the low temperatures attained during metamorphism, the assumption of equilibrium yields results consistent with field observations and phase relations.

Blueschists in major suture zones of China

Blueschist facies rocks and C‐type eclogites occur as allochthonous terranes or as small tectonic blocks in almost all the accretionary fold belts and major suture zones within the interior of China and along the Circum‐Pacific and Tethys‐Himalayan belts. At least forty blueschist localities have been described. Blueschists around the Pacific margins and the Himalaya‐Alpine belts are mostly Mesozoic in age; only those in Taiwan and New Caledonia are Cenozoic. Intracratonal blueschists of China are older than Late Permian, when the collision‐amalgamation presumably occurred. Several alleged Precambrian blueschist localities [Ye, 1987] are shown in the recently published Metamorphic Map of China. A Precambrian(?) blueschist belt in Anhui‐Hubei (central China) extends for about 2300 km. However, except for the pre‐Sinian blueschist terrane in Aksu, the age determination for Precambrian blueschist metamorphism elsewhere has not been confirmed. Characteristic features of Chinese blueschists include the following: (1) blueschist facies metamorphism predates the continental collision‐amalgamation, (2) most blueschists have undergone multistage metamorphism, commonly with increasing temperature and/or decreasing pressure for later events, (3) assemblages include sodic amphibole + epidote + albite + quartz + phengite + chlorite + sphene; lawsonite, jadeitic pyroxene, and aragonite are not common, and (4) protoliths are mainly mafic, pelagic, and clastic rocks. Blueschist facies metamorphism in China may be divided into two types on the basis of the imposed geothermal gradient, hence the progressive change in mineral assemblage. The rare lawsonite‐bearing blueschists produced at lower temperatures and higher pressures are confined to the Inner Mongolia and Yarlung Zangbo suture zones where aragonite and possibly jadeitic pyroxene + quartz occur, and the blueschists are associated with subgreenschist facies rocks. Most other blueschists of China formed at higher temperatures along the transitional blueschist‐greenschist facies boundary where epidote, sodic, and calcic amphiboles are ubiquitous, and the blueschists are interlayered with greenschist facies rocks. The common occurrence of intracratonal blueschists in major suture zones of China and elsewhere in Eurasia indicates pre‐Mesozoic subduction‐accretion of oceanic crust and flysch sediments. These zones of accretion represent growth around the periphery of major Precambrian cratons. This fact, together with the distribution of ophiolite belts and available paleomagnetic data, provides evidence for an extremely mobile history of plate movement in Eurasia. However, the Paleozoic intracontinental blueschists and ophiolites of China have formed before, rather than during, final closure between cratons. They do not usually mark the location of terminal sutures but are the result of earlier accretion and continental growth by subduction, underplating, and imbrication of oceanic materials similar to the accretion history of westernmost North America.

Clastic lawsonite, glaucophane, and jadeitic pyroxene in Franciscan metagraywackes from the Diablo Range, California

Textural relationships and mineral chemistry identify a detrital origin for jadeitic pyroxene and some lawsonite and glaucophane in Franciscan metagraywackes in the Diablo Range. These clasts differ from the neometamorphic (post-depositional) assemblage by having larger grain sizes and deformational features, including bending, cracking, and optical strain polarization. Large strain-polarized jadeitic pyroxenes contain inclusions of strained lawsonite and glaucophane, but none of these minerals has grown across clast/matrix boundaries. By comparison, neometamorphic lawsonite forms small unstrained tablets crossing the margins of large clasts and is associated with quartz + albite + chlorite + white mica in recrystallized metagraywacke matrices. Similarly, authigenic glaucophane forms discrete small prisms or euhedral outgrowths on large clasts of sodic amphibole. Jadeitic clasts in contact with metagraywacke matrices may be deeply embayed and show marginal recrystallization to fine-grained mixtures of albite + chlorite + quartz or internally to pumpellyite + albite. Backscatter electron imaging and microprobe analyses consistently show that the pyroxenes have extremely variable Jd contents. Within a single clast, compositional heterogeneity can be as much as Jd 50 Ac 39 -Aug 11 to Jd 95 Ac 4 Aug 1 . Because their texture, habit, and chemical variability are similar to those of metasomatic jadeites from tectonic inclusions in serpentinite at New Idria, southern Diablo Range, or to those of some jadeite-quartz associations in extensively recrystallized metabasite knockers, such a provenance is inferred for pyroxene clasts in the Franciscan metasediments. In metaconglomerates, pebbles containing pyroxenes (Jd 59-98 ) formed original clasts that were overprinted later by the post-depositional lawsonitic metamorphism. Metabasites at Pacheco Pass have compositionally variable Na-pyroxenes (Jd 30-92 ) which indicate tectonic emplacement of the igneous bodies within enclosing (in some cases, pyroxenefree) metagraywackes. The Diablo Range metagraywackes are weakly developed blueschist-facies assemblages of lawsonite + white mica + albite + chlorite + titanite ± glaucophane ± aragonite ± pumpellyite, which indicate minimum metamorphic conditions of P = 3.5-5.0 ± 0.5 kb at T = 100-200 °C.

High pressure metamorphic rocks and their tectonic environment in northeastern China

This paper deals with the high-pressure metamorphic rocks mainly of the Ondor Sum Group in Inner Mongolia and the Heilongjiang Group in the Jiamusi massif. A high pressure/low temperature metamorphic mineral assemblage of glaucophane schist facies and greenschist facies characterized by lawsonite, jadeitic pyroxene, glaucophane and pumpellyite has been recognized in the Ondor Sum Group. In northeastern China the high pressure metamorphic rocks are mainly distributed in the ancient intercontinental area, north of the Sino-Korean platform. A part of them is distributed on the outer margin of the Caledonian fold belt along the north edge of the Sino-Korean platform and it was dated at 445.6 ± 15 Ma by 39Ar/ 40Ar isochron method on glaucophane schist and at 509 ± 40 Ma by RbSr isochron method on whole rock of meta-basalt. These isotopic data indicate an early Paleozoic age of the subduction on the north edge of the Sino-Korean platform. The other part is the Heilongjiang Group of Jiamusi massif on the outer margin of the Xingkaiian fold belt. The Heilongjiang Group was intruded by a granite of 460 Ma (KAr). The lithological feature of the high pressure metamorphic rocks of the Heilongjiang Group is possibly correlated to that in the Altay-Sayan district (540–900 Ma). These facts suggest a probable Sinian-early Cambrian subduction.

Metamorphic evolution of the Eiksunddal eclogite complex. Western Norway, and some tectonic implications

The metamorphic evolution of the Western Gneiss Region of Norway (WGR) can be evaluated from examination of eclogites formed as an integral part of the gneiss terrain (ie. in situ). Petrologic data from one of the larger “country-rock” eclogites within the northwestern part of WGR have been used to construct a P, T, t-path for this crustal volume. Textural relations as well as mineral composition and zonation suggest an early period of amphibolite facies conditions, followed by movement along a very steep positive P/T-gradient towards very high pressure (P≥25 kb) and moderate temperature (T≈600° C). Orthopyroxene coexisting with garnet at such conditions was poor in Al (<0.5 wt% Al2O3) and is preserved in the cores of orthopyroxene grains in garnet websterites. The increasing Al-content of orthopyroxene towards garnet, and increasing edenite substitution rimwards in zoned amphiboles, suggest a period of decompression accompanied by heating, towards the metamorphic maximum at T≈ 750° C, P≈20kb. Garnet zoning reflects a general prograde history, but the reactions that control the zoning are insensitive to pressure changes. Jadeitic pyroxenes may have formed in the gneisses at P-max, but would revert to plagioclase subsequent to the metamorphic maximum, giving rise to the observed mineralogy. Further decompression and cooling continued towards high-rank amphibolite facies conditions. This P, T, t-path reflects crustal thickening to 80–90 km during Caledonian time. The uplift path is qualitatively similar to theoretically derived paths in which uplift is assumed to be controlled by erosion. However, uplift following the metamorphic maximum may have been accelerated by a period of suture progradation.

Possible hydration anomaly in the upper mantle prior to Red Sea rifting: Evidence from petrologic modeling of the Wadi Natash alkali basalt sequence of eastern Egypt

Basalt flows of ∼90-m.y. age occur in the Wadi Natash area of the Eastern Desert of Egypt and consist of an assemblage of alkali-olivine basalt (AOB), hawaiite-mugearite (HM), and benmoreite. The Wadi Natash occurrence is one of several upper Mesozoic to lower Cenozoic volcanic suites that surround the Red Sea and may be related to early mantle activity in the area prior to later rifting. The most mafic rock type at Wadi Natash is AOB, which could not have equilibrated with the mantle because of low Ni contents (50 ppm). The AOB9s presumably formed by extensive olivine fractionation from a primary picrite. The HM9s could have fractionated from the AOB9s by separation of a low-Ti amphibole. Alternatively, both suites may have formed by melting of an eclogitic mantle, with greater melting of jadeitic pyroxene contributing to the formation of the HM9s than to that of the AOB9s. The benmoreites apparently formed by equilibration of high-Ti (kaersutitic) amphibole with AOB. The Wadi Natash alkali basalt suite was probably generated from a mantle that contained high, and possibly varying, amounts of water. Mantle hydration is consistent with observations of the development of small extensional basins in the Red Sea area during the Cretaceous and may be an expected precursor to rifting of the type that now characterizes the Red Sea.

Relative geobarometry from high-pressure rocks of quartzofeldspathic composition from the Sesia Zone, Western Alps, Italy

In the Sesia Zone of the Western Alps, Italy, early Alpine blueschist to eclogite facies metamorphism of rocks of quartzofeldspathic composition has produced the same high-pressure assemblage of; quartz, Na-pyroxene, Na-amphibole, paragonite, phengite, zoisite, garnet, magnetite, sphene and Fe-sulphide (=the QFS assemblage) over an area (> 150 km2. Relative gradients in pressure and temperature over this region are reflected in the variations in mineral chemistries of the individual phases of the quartzofeldspathic assemblage through continuous reactions. Mineralogical discontinuities do not occur in the QFS assemblage of this region. Increases in the Jadeite content of the pyroxenes (XJd 0.48 to XJd 0.93) and in the glaucophane content of the amphiboles (XGl 0.89 to XGl 0.96) occur from the southwest to the northeast of the region studied. Analysis of coexisting garnets and pyroxenes indicate that the compositional variation of amphiboles and pyroxenes is associated with a decrease in the grossular component of the coexisting garnet. Zoned pyroxenes and garnets, together with the regional trends in mineral chemistries suggest that the evolution of the QFS assemblage with increasing pressure may be modelled by pressure-sensitive continuous reactions in which amphibole, zoisite and the more jadeitic pyroxene constitute the high-pressure assemblage. Chemographic constraints permit the positioning in pressure/temperature space of the compositional isopleths of those model continuous reactions involving these phases which meet the textural and chemical criteria observed in the natural assemblages. The low dP/dT slope (−20 bars/° C) of these isopleths causes the continuous reactions to be useful for geobarometric calculations at pressures above the absolute breakdown of albite to jadeite plus quartz. In addition the pseudobinary loops for the other continuous reactions which are potentially useful geobarometers and involve either the NaAlCa−1Mg−1 exchange or the MgCa−1 exchange are calculated. Comparison of mineral chemistries with the isopleths yields a relative barometric scheme for the localities studied. With these barometric observations, it is possible to show that the P-T path which the Sesia body travelled towards the final recorded state was one of increasing pressure. Other blueschist and eclogite occurrences from Syros and Sifnos which contain rocks of quartzofeldspathic composition are also examined.

Glaucophane and pyroxene breakdown reactions in the Pennine units of the Eastern Alps

Abstract New occurrences of crossite and jadeitic pyroxene are described from a thick metabasite unit within the upper levels of the Peripheral Schieferhülle in the Tauern Window, Austria. Unusual textures are preserved which provide evidence for the reactions and mechanisms involved in the breakdown of crossite and jadeitic pyroxene. Zones of albite and chlorite, produced by reaction between crossite and paragonite, have been preserved due to sluggish reaction kinetics during decompression from the blueschist to the greenschist facies. The zonal sequence is interpreted in terms of chemical potential gradients in Na, Mg and Al, which have been established by overstepping the equilibrium boundary. Breakdown textures of jadeite‐acmite pyroxene to a symplectite of albite + hematite + actinolite, and of crossite to talc and actinolite are also described.The occurrence of crossite and jadeitic pyroxene at high levels within the Peripheral Schieferhülle implies that even upper levels of the structural sequence have undergone blueschist facies metamorphism with pressures in excess of 8 kbar during the Alpine collisional event.

Low-temperature recrystallization of Franciscan greywackes from Pacheco Pass, California

AbstractLow-temperature metamorphism of the Franciscan complex at Pacheco Pass occurs at P-T conditions near the so-called jadeite isograd. Along Highway 152 in the Diablo Range, from west to east, four distinct prograde metamorphic zones are defined: (I) albite-quartz zone, (II) albite-rich, albite-quartz-clinopyroxene assemblage, (III) clinopyroxene-rich, albite-quartz-clinopyroxene assemblage, and (IV) clinopyroxene zone. Metamorphic pyroxenes are restricted to zones II, III, and IV in metagreywackes, but metabasites contain augite-rich pyroxenes even in zone I. With increasing grade, the compositions of pyroxene in metagreywackes change in XJd from nearly 100 in zone II to about 60 in zone III. The diopside component has little effect on the XJd of pyroxene. The apparent change of XJd reflects differences in pressure and temperature rather than in bulk rock composition. In metagreywackes, at the onset of zone II, albite breaks down to form Jd100 according to the reaction Ab = Jd+Qz. With increasing grade, this reaction leads to less jadeitic pyroxene. From zones II to III, a continuous reaction is delineated: Qz+2 pyroxenes (Jd80Ac15Aug5)=pyroxene (Jd60Ac30Aug10) + Ab.The metamorphic temperatures are estimated to be about 170°C in zones I and II, and about 230°C in zones II and III. The temperature variation recorded in a single rock is probably less than 30°C The pressure estimate depends on the choice of experimental data for the jadeite-albite-quartz curve; it ranges from 4.5 to 8 kbar. A gently folded thermal structure of the Cretaceous subduction zone metamorphism is postulated for this area.

Structural history of high-pressure metamorphic rocks in the southern vanoise massif, french alps, and their relation to alpine tectonic events

A nappe of amphibolite-facies metamorphic rocks of pre-Permian age in the southern Vanoise massif (the Arpont schist) has been affected by an Alpine HP/LT metamorphism. The first mesoscopically recognizable deformation (D 1) post-dated the high-pressure peak (jadeitic pyroxene + quartz, glaucophane + ?lawsonite), and was associated with glaucophane + epidote. D 1 produced a flat-lying schistosity and a NW-trending glaucophane lineation, and was probably associated with nappe displacement involving NW-directed subhorizontal shear. D 2 formed small-scale folds and a foliation associated with chlorite + albite. The changing parageneses during the period pre-D 1 to D 1 to D 2 suggest decreasing pressure, so that the deformation appears to have been related to the uplift history, rather than to the process of tectonic burial. D 2 was followed by a static metamorphism (green biotite + chlorite + albite), possibly of Lepontine age. SE-directed backthrusting and folding (D 3), and later differential uplift along steep faults, took place under low-grade conditions.

Implications to garnet-clinopyroxene geothermometry of non-ideal solid solution in jadeitic pyroxenes

Garnet-clinopyroxene geothermometry has been applied to numerous rock types from the blueschist and eclogite terrain of the Sesia zone in the Western Alps. Discrepancies in apparent temperatures of up to 600° C have been found suggesting substantial deviation from ideal behaviour of the garnet-clinopyroxene equilibria in certain assemblages. Assemblages containing jadeitic pyroxenes rather than omphacitic or diopsidic pyroxenes yield very low KD (2.0) values and correspondingly high apparent temperatures (> 1,000° C). The garnets coexisting with high-Na pyroxenes have compositions similar to those coexisting with omphacitic pyroxenes. Departure from ideal behaviour of these garnets is likely to be minor and is accomodated in the formulation of the geothermometer. Numerous examples of coexisting garnet-clinopyroxene pairs indicate that there is no obvious relationship of KD with jadeite content in pyroxenes with jadeite content less than ∼70%, but at higher values, KD varies inversely with the jadeite content. The dependence of KD upon the jadeite content in the high-Na pyroxenes is believed to be due to the preference of M2 sites for Fe2+ in the enstatite-jadeite substitution (Fe2+MgNa−1Al−1). This substitution is usually very limited and has no demonstrable effect upon X Fe Cpx when the M1 (Fe2++Mg) content of the pyroxenes is high as in omphacitic and diopsidic pyroxenes. However, when the M1 (Fe2++Mg) content is low, as in jadeitic pyroxenes, the non-ideality of the enstatite type exchange has a marked effect on X Fe Cpx and consequently on garnet-clinopyroxene calculations.

Metamorphic History of a High-Grade Blueschist Exotic Block from the Franciscan Complex, California

A high-grade blueschist tectonic block from the Franciscan Complex of the northeast Diablo Range shows evidence of three episodes of retrograde blueschist facies metamorphism ± deformation developed under progressively declining P-T conditions. The first retrograde metamorphism involved formation of an outer rind of actinolite + chlorite + rutile ± phengite, and the growth of coarse-grained chlorite + pumpellyite within the block. During the second event the rind and outer edge of the blueschist were folded, sheared and fractured, and primary glaucophane within the blueschist was replaced by albite, medium-grained chlorite, and glaucophane-crossite. The third retrograde metamorphism was marked by the pseudomorphic replacement of rind actinolite by aragonite and quartz. Aragonite also crystallized extensively within th block, accompanied by lawsonite, chlorite, jadeitic pyroxene, and crossite; this last mineral assemblage is identical to that of the surrounding Franciscan metasedimentary rocks. Features characteristic of the first and second retrograde metamorphic events are readily observed in other high-grade tectonic blocks of the Franciscan Complex and the correlative Otter Point Formation of Oregon. In contrast, evidence of a third retrograde metamorphism, matching that of presently associated Franciscan and Otter Point rocks, has been found in some but not all blocks examined so far. The high P/T conditions of prograde metamorphism and the available metamorphic age determinations suggest that the tectonic blocks originally formed in a pre-Franciscan subduction zone setting. Fragments of blueschist and eclogite from this metamorphic terrain were tectonically incorporated in a serpentinite diapir, and they developed alteration rinds through interaction with the enclosing ultramafic rock. The available data suggest that the examined exotic block and at least some others were transferred to the Franciscan as detritus from a body of serpentinite that reached the earth's surface. Such blocks were then resubducted and metamorphosed along with their presently associated sedimentary sequences.

High pressure metamorphism in Ghiaros Island, Cyclades, Greece

Summary The various rock types encountered in Ghiaros island have suffered an episode of progressive regional metamorphism which produced high pressure low temperature assemblages with Na-pyroxene and glaucophane. Later in their metamorphic history the rocks were affected by retrograde metamorphism. Glaucophane crystals became continuously zoned and sodic pyroxene crystals developed a patchy composition. Zoning in amphiboles extends from glaucophanitic cores through intermediate Na-Ca winchite compositions to actinolite rims. The compositional differences observed between the various patches in individual pyroxene crystals are interpreted to reflect contrasting P-T conditions which drive the continuous reaction jadeitic pyroxene + quartz ⇋ albite + acmitic pyroxene + quartz, to progressively less jadeite-rich pyroxene.

The experimental determination of activities in disordered and short-range ordered jadeitic pyroxenes

The activity of the jadeite (NaAlSi2O6) component in synthetic pyroxenes on the binary join NaAlSi2O6-CaMgSi2O6 has been measured at 600° C by equilibration of pyroxene with quartz and high albite in hydrothermal experiments. The results are in good agreement with recent calorimetry (Wood et al. 1980) for pyroxenes not close to the Jd50Di50 composition for which a considerable degree of short range ordering on the omphacite scheme is indicated by the activity measurements. For compositions close to Jd50Di50, the activity of the jadeite component is approximately equal to its mole fraction.

Compositions of aqueous fluids in equilibrium with pyroxenes and olivines at mantle pressures and temperatures

Solubility experiments were performed at 30 kbars in the system Mg2SiO4-SiO2-H2O, and at 20 and 30 kbars on omphacitic pyroxene-water mixtures. They confirm that the solubility of the forsterite component in aqueous fluids remains rather low (up to 5 wt.%), whereas the solubility of the SiO2 component from solids of appropriate SiO2-rich compositions in the system Mg2SiO4-SiO2-H2O increases with temperature up to some 75% at 1,100° C. At this temperature a simplified harzburgite consisting of forsterite and enstatite coexists with a fluid containing about 35% (MgO+SiO2). Hydrous fluids coexisting with omphacitic clinopyroxenes leach sodium silicate component from the solid leaving less jadeitic pyroxenes behind. Most interestingly, the amount of sodium leached at constant temperature increases with decreasing pressure. Comparison of the results with previous solubility studies in the system K2O-MgO-Al2O3-SiO2-H2O indicates that hydrous fluids in the mantle must be alkaline rather than silicanormative. Alkali metasomatism caused by such fluids would lead to potassium enrichment in deeper portions of the upper mantle and to sodium enrichment at shallower levels, where amphiboles become stable. This K/Na fractionation in the upper mantle may explain the generation of K-rich or of Na-rich magmas through partial melting at different depths.